It will be wierd WWI was a slaughter war. So... either change of topic, which risks, or... the end of series.
Casulties of WW1: about 11.000.000 (11-15 mio estimated, without further casualties)
Casulties of WW2: about 60.000.000 (60-90 mio estimated, without further casualties)
Considering numbers, WW2 was more of a slaughter war. WW1 is still considered by historians that altered the lifes of many countries to come.
There is indeed risk involved taking topics which have not been fully explored to gameify real-life stories and events to fit the mold of intended game-play of a video-game. One never thought WW1 would be an interesting topic for a FPS, considering the general beliefs of trench warfare. WW1 also had quite a few war changing mechanics, e.g. the introduction of the Lincolnshire boiling water on wheels (also known as tank), and air bombardment, which are less explored in WW1 video-games. Yet a popular company managed to adapt WW1 to a FPS, albeit a bit artistically bending the history to their will and the result does look rather fun.
And it seems to pay off for that company, considering the pre-ordering numbers being rather high and other companies intending to explore the WW1 theme.